Dispersive Mixing Zone

Tasks — Breaking down solids, polymer particles, and fluid droplets 

The dispersion effect is dependent on the shear stress as well as on the duration of the load. Which of the two variables has the most influence depends on the material to be dispersed [16]. On the basis of tests on a single screw extruder, Martin has demonstrated that the 

Together with a sufficient residence time in the stress zone, effective dispersion is also encouraged by 

Wide-disc kneading blocks achieve the most effective dispersion because they generate the greatest shear stresses. This also leads to high energy input, however, which increases the melt temperature and reduces the viscosity of the predominantly shear-thinning polymer melts, which in turn leads to a reduction of shear stress. Therefore, dispersive mixing is an optimization problem. 

The purely mechanical dispersion of the agglomerates can be assisted by the addition of dispersion aids (e.g., amide, polypropylene or polyethylene waxes, metal carboxilates, organic fatty acids and their esters), which wet the particle surfaces, thus reducing the interaction between them (see Fig. 4.14 and Chapter 9) [19]. This significantly lowers the shear stresses required to disperse the agglomerates. 

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For any given size of mill there is a minimum volume of polymer per unit width of roll, below which no dispersive mixing occurs. Increasing the amount of material above this minimum level increases the high shear zone in the polymer, increasing dispersive mixing. There is an upper limit however, above which a circulatory flow develops at the entrance to the nip region and no further improvement occurs. 

The annular centrifugal contactor is designed so that each stage achieves the required mixing and subsequent separation. Besides the conditions specified above, at least five additional factors must be considered. First, to allow the dispersion to flow by gravity from the mixing zone into the rotor, stationary radial vanes under the rotor must dissipate the rotational velocity of the dispersion. Second, the rotor inlet (RI) must be small... 

The bottom of the mixing zone contains stationary vanes attached to the bottom of the contactor housing. These vanes stop the rotation of the dispersion that is created in the annular gap and allows the dispersion to flow under the rotor. The opening into the rotor is located at the center of the bottom surface of the rotor. If the bottom vanes were not there, the dispersion would back up in the annular mixing zone and flow out via the lower collector ring. If this happened, a breaking dispersion would flow out the less-dense-phase exit port. 

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